Universal serial bus (USB), a new I/O standard promoted by main computer industrials, has been widely accepted by computer market. Since the USB port unifies many traditional I/O ports, such as serial ports, parallel ports, and PS/2 ports, into a single specification, it simplifies the usage of I/O ports. According to the specification, the USB port includes four wires, in which two wires convey digital information from port to port. The other two wires of USB port transmit electric power to drive peripheral devices connected thereon. For expanding the number of peripheral devices communicating with USB ports, many USB hubs may couple together to a host root hub, namely a computer, to build up a Tier-Star connection.
Typically, the USB hub includes at least a USB port and a direct current (DC) socket. In bus power mode, the host root hub transmits electric power through the two power lines of the USB port to drive the USB hub. Similarly, the USB hub transmits electric power to drive peripheral devices connected with it. In the present USB specification, the two power lines of USB port only convey electric current in about 100 mA. For some low power-consuming devices, this electric current is sufficient. However, for some high power-consuming devices, this electric current would not be able to drive them. In the cases of high power-consuming devices, the USB hub must be operated in external power mode by importing electric power through the DC socket. Thus, by selectively employing the USB port and DC socket, the USB hub would always drives the peripheral devices connected thereon.
Although the USB hub receives electric power selectively from the USB port or DC socket, it exits a problem of electrical interference, which is due to using the USB port and DC socket at the same time. When the electrical interference happens, the USB hub has no schemes to distinguish the two power modes, so that some errors or even more serious damages may happen. Therefore, there is a huge need to prevent the USB hub from occurring the electrical interference.